*Main> :k []
[] :: * -> *
*Main> :k State
State :: * -> * -> *
*Main> :k State Int 
State Int :: * -> *
*Main> :r
[1 of 1] Compiling Main             ( lec11.hs, interpreted )

lec11.hs:24:18: error:
    • Couldn't match expected type ‘(b, s)’
                  with actual type ‘State s b’
    • In the expression: f y
      In the expression: let (y, s) = g x in f y
      In the first argument of ‘S’, namely
        ‘(\ x -> let (y, s) = g x in f y)’
    • Relevant bindings include
        s :: s (bound at lec11.hs:23:23)
        x :: s (bound at lec11.hs:23:10)
        f :: a -> State s b (bound at lec11.hs:22:15)
        g :: s -> (a, s) (bound at lec11.hs:22:12)
        (>>=) :: State s a -> (a -> State s b) -> State s b
          (bound at lec11.hs:22:3)
   |
24 |               in f y 
   |                  ^^^
Failed, no modules loaded.
*Main> :t () 
() :: ()
*Main> runState 0 (label tree1)
(Node 0 (Node 1 (Leaf 2) (Leaf 3)) (Node 4 (Leaf 5) (Leaf 6)),7)
*Main> runState 7 (label tree1)
(Node 7 (Node 8 (Leaf 9) (Leaf 10)) (Node 11 (Leaf 12) (Leaf 13)),14)
*Main> runState 7 (label tree1)
(Node 7 (Node 8 (Leaf 9) (Leaf 10)) (Node 11 (Leaf 12) (Leaf 13)),14)